On the glass forming ability of bulk metallic glasses

During the past few years, a number of multicomponent metallic alloys have been found to exhibit an excellent glass forming ability when cooled from the melt with rates of less than 100 K/s and as low as 1 K/s. This permits the formation of bulk amorphous metallic materials with sizes of up to several centimeters in the smallest dimension. In this article several factors are discussed that contribute to the high glass forming ability of these materials. Results based on levitation experiments, differential scanning calorimetry, viscosity, atom probe field ion microscopy-, and electron microscopy are presented. The thermodynamics as well as the kinetics of bulk metallic glass forming liquids, expressed by the Gibbs free energy and the viscosity, respectively, favor glass forming ability. In addition, phase separation in the undercooled liquid state becomes an important factor in understanding the nucleation process of crystals. Finally, the role of the different species in the multicomponent material is discussed based on solid state reactions in multicomponent systems.